Why don't browsers collide traffic?

Something hit me a minute ago, and I'd like to clear it out for my own sake.

Assume that I have two applications running on one machine, using the same network port. Will traffic sent to that port from outside be routed to both applications? If so, how come Firefox and Safari can co-exist nicely? And if not, why do we not all run games on port 80 to avoid NAT punch-through issues?

Port 80 is the REMOTE port the browsers are accessing. When they open a connection, the remote server is told which port to use to respond. The browser simply requests an unused port from the OS, and uses that for communication. See http://www.camden411.com/tcpipfaq/ports.html